Components with nanotips, the tips of which have a radius of less than 10 nm, and more specifically of less than 1 nm, are used, inter alia, as field emission components in flat panel displays, high-performance microwave tubes, as quantum-effect components and as scanning tips in scanning electron microscopy. More recently, nanotips have also been used as part of biosensors.
There are various known methods for producing nanotips. One method known from U.S. Pat. No. 5,233,263 is to etch silicon in such a way, using a mask, that truncated cone are firstly produced. The outer surface of the truncated cone is then given a concave shape by an isotropic etching process. The tip is then formed by repeatedly oxidizing the truncated cone and removing the oxide layer. Alternatively, the tip can also be formed by sputtering a precious metal.
In order to avoid the oxidation step for sharpening the tip, DE 195 01 387 proposes that the tips be shaped by etching until a full undercut is achieved. FIG. 1 illustrates schematically the method described in DE 195 01 387. A mask 120 is used which is perfectly balanced on the apex of tips etched from substrate 110, even after full undercutting. A plasma dry etch process is continued until the mask has been fully undercut by the lateral etching process and a sharp silicon tip has been formed. Curved arrows in FIG. 1 illustrate the etching process when a plasma dry etch method is used.
Reactive ion etching (RIE) for producing nanotips is presented in the publication by W. Mehr et al. entitled “Ultra Sharp Crystalline Silicon Tip Array Used As Field Emitter” Microelectronic Engineering 30 (1996), pp. 395-398. The basis is a silicon wafer, on the surface of which an SiO2 layer was thermally grown. Using a mask, the oxide layer is structured so that, for example, a circular oxide layer remains at the positions where the tips are desired. After removing the mask, the wafer is subjected to another RIE process. The etching process is then anisotropic. The different etching rates of SiO2 and Si, in a ratio of approximately 1:5, are exploited. The etching process is continued until the mask has been entirely removed, then stopped. What remains is a sharp tip made of silicon.